Drip irrigation systems have now been known and used for many years on an increasingly large scale. Whilst some of the original proposals for the use of drip irrigation involved the laying of the irrigation pipes in the ground, this subterranean form of drip irrigation has not been generally adopted and most drip irrigation systems involve locating the drip irrigation conduits above ground. One of the main reasons for the hitherto relative lack of success in such subterranean drip irrigation systems is generally attributed to the fact that such systems are prone to blockage by plant roots which tend to grow in the direction of the water outlets of the system, thereby tending to block these outlets.
There have in the past been proposals to overcome this problem, for example by periodically injecting into the throughflowing irrigation flow a suitable herbicide. Whilst such herbicides can, in appropriate doses, selectively inhibit root growth, and thereby prevent or limit blockage of the water outlets by roots, the growth of the plant as a whole is not deleteriously affected. If, however, for one reason or another the maximum dose of the herbicide is exceeded then, quite clearly, the plant as a whole may be damaged or destroyed. The difficulties inherent in ensuring that the dosage of the throughflowing irrigation flow does not exceed a prescribed level have limited the adoption of this proposal.
It has furthermore been proposed (U.K. Patent Serial No. 2174884) to tackle this problem by providing for the use of drip emitter units which are formed of a plastics material which is wholly or in part impregnated with a herbicide material which is capable of being released into the irrigation water over a considerable period of time. This proposal is, however, subject to several major drawbacks. Thus, it is known that the release of such herbicides is very much dependent on environmental conditions in general, and on surrounding temperature in particular. For example, a rise in ambient temperature of 10.degree. C. can very much increase the rate of release of herbicide from the plastic material in which the herbicide is impregnated and this could lead to an undesirably high concentration of the herbicide in the soil with consequent damage to the plants.
Furthermore, seeing that the release of the herbicide from the plastic material takes place continuously and, as indicated above, this release is significantly accelerated by any rise in temperature, it will be apparent that such release will also take place during storage and transport. In consequence, and prior to the laying of the drip irrigation system in the ground, the emitters and/or any other components of the system which consist of or include such herbicide impregnated components, must be suitably packaged and enclosed if it is to be ensured that by the time the system is rendered operational, the herbicide content of the system will not have fallen below required levels. This requirement for suitable packaging and enclosing which can, of course, involve the packaging and enclosing of many hundreds of meters of irrigation piping, can be clearly complicated and costly.
Furthermore, it will be realized that the size of the emitter unit clearly determines the initial concentration of the herbicide in the unit and, in consequence, the amount of herbicide in a relatively small unit will be significantly less than that in a relatively large unit. It will therefore be necessary to determine the concentration of the herbicide in the plastics material, bearing in mind the size and surface area of the unit to be manufactured, and this clearly complicates the manufacture of such units.
Finally, with a drip irrigation system employing such emitter units, once the concentration of the herbicide in the system falls below a predetermined minimum there is no alternative but to replace the system as a whole, and this clearly is time-consuming and expensive.
The problems referred to above and, to a limited degree, the suggested solutions thereof, whilst relating particularly to drip irrigation, also arise from time to time in connection with other forms of irrigation, to which the present invention is also directed.
It is an object of the present invention to provide a new and improved irrigation system wherein the disadvantages involved in the hitherto-proposed solutions are substantially reduced or overcome. It is also an object of the present objection to provide a method for the controlled dosing of an irrigation flow with a herbicide, in which the disadvantages referred to above are substantially reduced.